I have a xbox slim 4GB, 0401 FW, that has been mod which using the replacement PCB.
Dashboard 13416. It can run the backup games.
I only played the single player version, I am wondering Can i use the xbox live and play multiplayer games?
you'll have to update to 13599, id recommend flashing to stock first, then reflashing to LT1.9 or even waiting for LT1.91.
if i flashed the LT1.9 can i play with original games on xbox live?
You flash to stock the same way you flash to cfw...use your original fw dump.
It is advised to flash to stock by TX and c4eva, like I said latest dash 599 will let you play online